Add canonical examples for the marketing SDK pages

[Jonesy464]

Part of anvilco/marketing-site#3646 (SDK repos own their code examples; the central anvil-api-usage-examples repo is being sunset).

Renames the overlapping examples to the canonical names the marketing SDK pages source at build (the sourcing shipped with anvilco/marketing-site#3617 scans examples/ and kebab-cases each basename into a concept, so generate_markdown.py serves the generate-markdown page):

• fill_pdf.py -> fill.py
• generate_pdf.py -> generate_markdown.py (markdown-only; the HTML variant gets its own file)
• new generate_html.py
• create_etch_existing_cast.py -> create_etch_packet.py

The other examples (workflow, GraphQL, uploads, forge) are untouched. Content refresh along the way:

• The etch signer's field_id was the placeholder def456, which doesn't exist on the sample template; now signature (verified live).
• The etch packet now fills the template via add_file_payloads before sending, matching the JS/TS examples, and takes the signer email from SIGNER_EMAIL.
• Stale fill data cleaned up; long commented-out option blocks trimmed so the files read well on the marketing pages, which render the file contents directly.

Verification:

• isort, black, pylint, and pydocstyle all pass at the poetry.lock-pinned versions. mypy reports only the pre-existing EtchSigner false positive that also fires on untouched examples under a newer pydantic; CI's pinned env is unaffected.
• All four executed live against the API with a dev key: fill and both generates returned 200 with valid PDFs (confirming the sample template and its field IDs), and the e-sign mutation created a packet (run as a draft so no email was sent; the committed file keeps the client's defaults: is_test=True, is_draft=False).
• The marketing site's deriveLanguageExamples resolves the four canonical concepts from this directory.